1. Field
An aspect of the art of the invention relates to Micro Electro Mechanical Systems (MEMS) device.
2. Description of the Rerated Art
An optical switch used for optical systems, such as OADM (Optical Add Drop Multiplexing), is a key device that is advantageous in high speed and reduction in number of parts because of a switching operation with direct light without conversion into an electric signal. In particular, small size and integration are possible for a multi-channel micro-mirror device using a micro-machine technology, which is actively developed by companies.
Specifically, an optical switch using a Vertical-MEMS device with a silicon on insulator (SOI) substrate has been developed. Japanese Laid-open Patent Publication No. 2006-247793 (Patent Document) discuss that the use of a vertical comb electrode as an actuator enables a large deflection angle with a low voltage in the MEMS device.
FIGS. 6A to 6E are a diagram showing examples of the structure of a conventional MEMS device, FIG. 6A is a top view thereof, FIG. 6B is a cross-sectional view with AA in FIG. 6A, FIG. 6C is a cross-sectional view with BB in FIG. 6A, FIG. 6D is back view thereof, and FIG. 6E is an enlarged view of a portion P in FIG. 6B and FIG. 6C. An MEMS device 15 shown in FIG. 6A to FIG. 6D comprises a rotating unit 11 having a mirror 11A, frames 12A and 12B, a hinge 13A that connects the rotating unit 11 to the frame 12A, a hinge 13B that connects the frames 12A and 12B, and comb electrodes 14A to 14D that receive voltages for generating electrostatic force.
As shown in FIG. 6B and FIG. 6C, the MEMS device 15 is shaped by processing an SOI substrate structured by sandwiching an insulating layer 18 containing SiO2 by two silicon layers 16 and 17, with an etching technology, etc. As shown in FIG. 6A-6E, the rotating unit 11 and the hinge 13A comprise an insulating layer 16, the frame 12A comprises the silicon layer 16 and further partially comprises a silicon layer 17 and the insulating layer 18, and the hinge 13B and the frame 12B comprise the silicon layers 16 and 17 and the insulating layer 18.
The hinge 13A supports the rotating unit 11 so as to enable the rotation of the rotating unit 11 around the hinge 13A as an axis thereof by electrostatic force produced with voltages applied to electrodes 14A and 14B, which will be described later. The hinge 13B supports the frame 12A so as to enable the rotation of the rotating unit 11 around the hinge 13B as an axis thereof together with the frame 12A and the hinge 13A by electrostatic force produced by electrical fields applied to electrodes 14C and 14D, which will be described later.
The two comb electrodes 14A and 14B (or comb electrodes 14C and 14D) with arrangement of a plurality of combs in facing directions are formed onto the upper and lower silicon layers 16 and 17 in the diagram, thereby structuring vertical-comb electrode actuators 41 (42). In this case, as shown in FIG. 6A to FIG. 6D, the comb electrode 14A is formed on side surfaces of the silicon layer 16 of both edge areas of the rotating unit 1 facing the silicon layer 16 of the first frame 12A with arrangement of a plurality of combs directed to the frame 12A side in the direction of an AA′ axis (top side and bottom side), the comb electrode 14B is formed on side surfaces of the silicon layer 17 forming the frame 12A with arrangement of a plurality of combs in facing directions of the combs of the comb electrode 14A, and a comb electrode actuator 41 is formed a pair of the comb electrode 14A and the comb electrode 14B (refer to FIG. 6B).
Similarly, as shown in FIG. 6A to FIG. 6D, the comb electrode 14C is formed on side surfaces of the silicon layer 16 of both edge areas of the first frame 12A facing the second frame 12B with arrangement a plurality of combs directed to the frame 12B side in the direction of an axis BB′ (left side and right side), the comb electrode 14D is formed on side surface of the silicon layer 17 of the both edge areas of the second frame 12B facing the first frame 12A with arrangement of a plurality of combs in facing directions of the combs of the comb electrode 14C, and a comb electrode actuator 42 comprises a pair of the comb electrode 14C and the comb electrode 14D (refer to FIG. 6C).
Herein, the vertical-comb electrode actuator 41 is formed on both sides of the rotating unit 1 in the direction of the AA′ axis, and a voltage is supplied with a cooperative operation. Namely, the rotating unit 1 is rotated only at an angle corresponding to the electrostatic force generated by supply voltages around the hinge 13A as the rotating axis formed-along the BB′ axis. Similarly, the vertical-comb electrode actuator 42 is formed on both sides of the rotating unit 1 in the direction of the axis BB′, and a voltage is supplied with a cooperative operation. Namely, the rotating unit 1 is rotated together with the frame 12A and the hinge 13A only at an angle corresponding to the electrostatic force generated by supply voltages around he hinge 13B as the rotating axis formed along the AA′ axis. As a consequence, an angle of the mirror 11A formed to the rotating unit 1 is given and a reflection angle of light incident on the mirror 11A can be deflected depending on the angle of the mirror 11A.
However, in the MEMS device 1 with the above-mentioned structure, the upper and lower comb electrodes 14A and 14B (14C and 14D) sandwiching the insulating layer 8 generally have the potential difference therebetween. FIG. 6E is an enlarged view of a boundary part between the silicon layers 16 and 17 sandwiching the insulating portion 18 on the frames 12A and 12B where the electrodes 14B and 14D are formed on the silicon layer 17. Referring to FIG. 6E, via the insulating layer 18, there is the potential difference between the upper silicon-layer 6 in the drawing and the silicon layer 17 where the electrodes 14B and 14D are formed.
The insulating layer 18 usually comprises a greatly thin film of 1 μm or less. When there is the potential difference between the silicon layers 16 and 17 via the insulating layer 18 as the thin film, electric discharge is easily generated on the insulating layer 18. When the electric discharge is generated between the silicon layers 16 and 17, the electrostatic force to be generated is changed. Thus, there is a problem of deterioration in stability of the rotating operation of the rotating unit 11, i.e., stability of the angle setting of the mirror 11A.
At this point, the increase in thickness of the insulating layer 18 can suppress the generation of the electric discharge. However, as the insulating layer 18 is thicker, the thickness is distributed. Further, the warpage of the wafer itself, as the base of the SOI substrate becomes large, and there can be thus a trouble upon manufacturing the device with high quality. Alternatively, the potential difference applied to the comb electrodes 14B and 14D can be also reduced as a countermeasure for suppressing the electric discharge. Primarily, if the potential difference is not fully applied, a required inclination angle of the mirror 11A cannot be obtained.
Patent Document discusses the technology for improving the alignment accuracy upon forming the pair of comb electrodes. However, the technology for suppressing the electric discharge generated as mentioned above is not disclosed.